Tag: Flag Checker

Flag Checker

Flag checkers are a common type of challenge in the Reverse Engineering category.

An executable or script called a flag checker takes the player's input and outputs whether that input matches the flag.

These challenges may also be called "crackmes" in a broader sense. (automatically translated from Japanese)

·Updated May 3, 2026

Flag Checker Challenges

C# File-based app

Daily AlpacaHackTopic: .NETReleased: Oct 2, 2026

72 solves

tan90909090

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Thanks to the File-based apps feature added in .NET 10, we can now run a .cs file directly!

Beginner Hint 1: Implicit using directives feature in C#

The Implicit using directives feature was added in .NET 6. By default, several namespaces are automatically globally using.
Therefore, using System; can be omitted in chal.cs. Please be careful when investigating various APIs.

Beginner Hint 2: How to handle the Brotli compression format

There are various ways to handle the Brotli compression format, such as using .NET (similar to chal.cs) or JavaScript. In JavaScript, we can use the zlib module in Node.js, or the DecompressionStream interface in certain browsers. If you are unsure how to decompress Brotli-compressed data, you might want to try these languages.